1. Field of the invention
The present invention relates to a device for fabricating a thin layer on a silicon wafer. More particularly, the present invention relates to a wafer boat for use in a consolidation process after a deposition of silica layer.
2. Description of the Related Art
In the prior art, oxide layers for the fabrication of a planar light-wave circuit are produced through a flame hydrolysis deposition process. The flame hydrolysis deposition includes deposition of oxide particles, such as SiO2, GeO2, P2O5, or B2O3 onto a silicon substrate or a quartz substrate, and then a subsequent consolidation process. In the process of the flame hydrolysis deposition, the source material is injected into a flame in order to form fine particles through hydrolysis reaction or the oxidation in the flame. The fine particles formed are coagulated through collision with each other while moving through the flame, and are deposited to the silicon substrate or the quartz substrate by thermophoresis. The particles deposited onto the substrate such a porous oxide layer having been deposited is mounted onto a wafer boat and introduced into a furnace to perform the consolidation process.
FIG. 1 is an illustration of a side view of a wafer boat 100 according to an embodiment of the prior art, and FIG. 2 is a cross-sectional top view taken along a line A–A′ of FIG. 1. As shown in FIGS. 1 and 2, a wafer boat 100 for consolidation of porous layer according to the prior art includes an upper plate 105, a lower plate 107 and four support rods 101. Alternately, three, five or even more support rods may be installed.
The rods 101 are fixed between the upper plate 105 and the lower plate 107. A plurality of fixture slots 103 are formed along each of the support rods 101 with equal distances to each other. A plurality of silicon or quartz wafers 110 on which the porous layers 113 are formed are mounted in the equally distanced fixture slots 103.
Because the porous silica layer 113 formed by the flame hydrolysis deposition has a tendency to be damaged or broken even through very light contact, or an impact from the exterior, careful attention should be paid to the handling of the wafer 110 when it is mounted on the wafer boat 100 for consolidation after the deposition process. In particular, the porous layer 113 may be damaged by contact with the support rods 101 while mounting the wafer 110 onto the wafer boat 100 after the deposition. Further, it may be damaged by movement of the wafer 110 due to the vibration or sway of the wafer boat 100 after mounting the wafer 110 to the wafer boat 100.
Also, when the consolidation process is terminated, there are instances when the porous silica layer 113 is formed in such a state that a portion thereof protrudes from the edge of the substrate 111. In this state, the thin layer protruding out of the substrate edge is extremely vulnerable to breakage even by the slightest impact, resulting in the broken particles falling down onto another wafer mounted underneath, which in turn causes other defects in the products.
In order to prevent the broken particles from falling down onto a lower wafer or to facilitate the easiness of handling of the wafer with the porous layer, a dummy wafer 120 (FIG. 3) may be employed.
FIG. 3 is a side view showing the dummy wafer 120 mounted to the wafer boat 100. Since the dummy wafer 120 has a diameter larger than that of the wafer 110 to be consolidated, particles having been broken off the porous silica layer 113 of the wafer 110 can be prevented from falling down to a wafer located underneath. Also, since a wafer is not properly supported by the support rods 120 when the wafer is small-sized, a dummy wafer 120 may be used.
Generally, a silicon wafer is used as the dummy wafer 120.
However, the surface of the dummy wafer will be oxidized leading to crystallization because the consolidation process is performed at a high temperature of about 1300° C. Fine crystals formed on the surface of the dummy wafer will fall down to another wafer located underneath, causing the defects in the oxide layer on the wafer. Thus, the dummy wafer should be replaced after a predetermined period. Alternatively, a dummy wafer made of SiC material may be used that becomes relatively less oxidized at a high temperature. However, such a solution increases production costs. Further, since the method utilizing the dummy wafer should employ a dummy wafer larger than the substrate to be consolidated, the size of the consolidation furnace increases accordingly. Also, in a case in which the mounting of the substrate is automated, a complicated wafer automation system is required so as to make use of the dummy wafer, and the operation of the automation system also will be complicated.